An immunoassay is proposed which utilizes a fluorescent dye as label rather than radioactivity. The assay may be applied to measure any analyte toward which an antibody may be generated and to which a fluorescent dye may be conjugated. The proposed assay would ultimately be developed to measure PCP in biological fluids. During trial development, which is the objective of this submission, application of the assay to measurement of thyroxine will be investigated. An important practical feature of the proposed assay is that interfering emission from specimen components will be substantially reduced. The latter is a problem which has precluded practical utility of fluorescent labels in routine immunoassays, particularly for measurement in urine specimens. Emission from components in solution will be effectively eliminated by the presence of quenching species, e.g. iodide, in the assay system. Fluor labeled analyte, competitively released from binding to analyte specific antibody by the presence of unlabeled analyte, is sequentially bound by fluor specific antibody. Fluor labeled analyte is not bound by fluor specific antibody when bound by analyte specific antibody. The fluor, rhodamine B, is fluorescent when bound to fluor specific antibody and is also protected from contact with the quenching species in solution when bound. Assay response is thus an increase in fluorescent intensity with increasing concentrations of analyte from standards and specimens. Assuming that antibody affinities in the vicinity of 10 to the 10th power/M can be realized, assay sensitivity will be limited by instrumental sensitivity which is in the vicinity of 10 to the minus 11th power M. Distinction of free from bound labeled analyte is attained without physical separation so that the assay will be rapid, convenient, and easily automatable.